MIDDLETOWN, United States (AFP) - Wildfires sweeping across California are threatening the US state's famed Sequoia trees, with firefighters scrambling to protect the national treasures.

The so-called Rough Fire, the largest of more than a dozen burning across northern and central California, has edged closer to the giant trees in recent days with firefighters scrambling to protect them.

"The fire has moved into a number of Sequoia groves in King's Canyon National Park and Sequoia National Forest and we are taking preventive measures to make sure nothing happens to them," park spokesman Mike Theune told AFP.

Of particular concern is the General Grant tree, the second largest Sequoia in the world. It stands 81.7m tall.

Theune said firefighters are monitoring the tree round-the-clock, spraying water and clearing the area around Grant grove.

"We have some of the best firefighters in the world working on this fire in order to protect these national treasures," he said.

Theune said crews had also installed a sprinkler system around the Boole Tree, the sixth-largest tree in the world.

Although the Sequoias, which are a major attraction for tourists worldwide, need low-intensity fires to reproduce, extreme heat like that from the Rough Fire is too much for the giants to handle.

The Rough Fire has burned 139,000 acres near Sequoia and King's Canyon National Parks. More than 3,700 firefighters are battling the blaze, which is 40 per cent contained, according to the US Forest Service.

Thousands of firefighters further north are also battling two fast-moving wildfires - the Valley Fire and the Butte Fire - that erupted over the weekend.

The blazes killed at least one person, forced the evacuation of more than 23,000 and destroyed more than 700 homes.

Of the two fires, the Valley one has been the most devastating and difficult to contain. Officials said 9,000 homes are still threatened in the area.

Middletown, a small hamlet north of San Francisco, was completely gutted by the flames that left an apocalyptic scene and shocked even seasoned firefighters with its speed and strength.

Residents trickled back to the town on Tuesday to check on their still smoldering homes. Officials said at least 400 homes and businesses had burned to the ground.

"Everyone in here could tell you a horror story you wouldn't believe," said Ashley Mayhew, manager at Hardester's, a market in the heart of Middletown that stayed open during the inferno.

She said desperate residents had flooded the store over the weekend, buying everything from water, groceries, tools and other supplies.

"People... were hosing down their houses to try to save them," Mayhew told AFP.

'GOING TO GET WORSE'

Fire officials said there was reason for hope in battling the Butte Fire, and that some residents of the San Andreas area should even be allowed back to their homes.

But although temperatures are expected to be much cooler this week than last, fire officials remain on alert due to hot conditions and landscape left bone dry by a four-year drought.

Governor Jerry Brown, who has declared a state of emergency in the affected counties, said on Monday he did not see an end to the fire season in the months ahead, blaming climate change in part for the blazes.

"It is going to get worse because of the nature of climate change," he told reporters.

John Wickstrom, engine captain with the US Forest Service, said crews were struggling to anticipate the path of the fires.

"With the conditions and the damage that's already done, we could be out here for weeks," he said.

For now, the fires have not affected vineyards in the Napa Valley.

"The pattern of the Valley Fire as it is now does not indicate an imminent threat to Napa Valley," said Cate Conniff, spokeswoman for the Napa Valley Vintners organization.

"So we have been very lucky." Firefighters this summer have also been battling wildfires in other West Coast states, including Washington and Oregon, that have stretched resources thin.

Officials say the wildfires, which have mobilised 30,000 firefighters, could be the costliest on record with more than US$1.23 billion (S$1.72 billion) spent so far.